The present invention relates to a method for automatically applying (placing) cut strips of material, preferably cut at a slant and having varying length tolerances, onto a tire building drum that is rotated at varying rotational speeds for splicing and manufacturing green tires. The initially endless strips of material are supplied to the tire building drum with a substantially continuous conveyor belt.
The present invention furthermore relates to a device for performing the method.
Such endless strips of material may be comprised of unvulcanized rubber reinforced with reinforcement elements extending at a slant and can be fed manually or automatically to the tire building drum. The continuous (endless) strip of material is supplied from a supply drum and cut to length with the aid of a cutting knife. The cutting knife always locates a gap between two of the reinforcement elements for the cut. Thus, cut material strips of different lengths result. The differences in length can be 10 to 20 mm. For strips of material without embedded reinforcement elements it is also possible that cutting tolerances occur.
When applying cut strips of material to a tire building drum, it may be desired to have in the final applied state a defined overlap of the ends or to have a blunt abutment of the ends. Subsequently, the latter case will be treated. When a strip of material is too long, an overlap results which represents an undesirable material accumulation. When a cut strip of material is too short, gaps between the ends result which cause undesirable material weaknesses in the finished tire. When applying the cut strips of material onto the tire building drum, they must therefore be compressed lengthwise in order to shorten them or they must be stretched in order to bridge the otherwise resulting gaps.
From German Patent 32 13 366 a method is known in which the endless strips of material are cut to a length shorter than the required length in order to subsequently stretch the cut strips of material to the desired final length over a drum of a corresponding circumference and diameter. This is disadvantageous because for each ply and each working step a different (stretching) drum must be used, respectively, the drum diameter must be changed.
From European Patent 0 164 709 a method is known in which the rotational speed of the tire building drum is adjusted corresponding to the feeding speed of the cut strip of material to be applied. In this prior art method, the actual length of the material strip is compared automatically to the nominal length that is determined by the circumference of the drum. The measurement of the length of the cut strip of material is carried out with a light beam/photocell controlled impulse counter whereby each impulse corresponds to a certain feed length. The ratio of actual length to nominal length is used to calculate a speed ratio which is supplied to a control device of the drive units for conveying the strip of material and driving the tire building drum. It is disadvantageous that the rotational speed of the tire building drum is adjusted as soon as the cut strip of material arrives thereat. Since the cut strips of material, due to the slantedly extending reinforcement elements, are cut in a diamond or rhomboid shape, pulling forces, resulting from the acceleration of the tire building drum, act on the cut strips of material first on the leading tips of the rhomboid material strips so that in this area a relatively high specific pulling load results which may cause a reduction of width of the cut strip of material.
It is therefore an object of the present invention to provide a method of the aforementioned kind with which non-uniform pulling loads and changes in width can be prevented and with which the ends of the material strips can be spliced in a controlled fashion on a tire building drum.